1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine and it particularly relates to an air-fuel ratio control system for an internal combustion engine which system has an evaporated-fuel purging system and which system has a learning and a control function.
2. Description of the Related Art
A known internal combustion engine includes an evaporated-fuel purge system having a canister for temporarily storing evaporated fuel. (The evaporated-fuel purging system temporarily stores fuel evaporated from the fuel tank, which tank is used for supplying fuel to the internal combustion engine. The evaporated-fuel purging system purges the stored fuel when a predetermined condition is met.) This engine further has an air to fuel ratio sensor located in an exhaust-gas passage used for discharging exhaust gas generated in the internal combustion engine. The engine that controls an air-fuel ratio so as to cause the air-fuel ratio to be a predetermined target air-fuel ratio. This controlling is executed by correcting a fuel injection quantity according to feed-back correction factors (FAF). The feed-back correction factors (FAF) are appropriately determined according to the variation of the detected air-fuel ratio. The fuel injection quantity is a fuel quantity being injected to the internal combustion engine for the combustion operation in the engine.
This engine may also include a control system for determining a basic fuel-injection-quantity used in a combustion process in the internal combustion engine. In this case, this determination is executed using an internal pressure of a surge tank provided in the internal combustion engine and using an engine rotation speed. The surge tank internal pressure is obtained from measuring the pressure by means of an intake-air pipe pressure sensor. In the internal combustion engine, this intake-air pipe pressure is used for determining the relevant intake-air quantity.
The above-mentioned determination of the basic fuel-injection-quantity is executed according to a predetermined basic fuel-injection-quantity map. However, the basic fuel-injection-quantity map may need an appropriate correction due to variation of the characteristics in the internal combustion engine. This variation occurs due to the prolonged passage of time in the engine. This variation may result from undesirable variations (degradations) including shift (variation) of characteristics in various sensors and actuators (for example, an injector) used in the engine.
To overcome the variations due to the prolonged passage of time, the above-mentioned control system for determining the basic fuel-injection-quantity has a function to learn an effect resulting from the variation due to the prolonged passage of time. The effect may degrade the above-mentioned air-fuel controlling operation efficiency. The control system has learning values resulting from the learning, which learning value are used for eliminating the effects resulting from the variation due to the prolonged passage of time. These learning values may be respectively provided for a plurality of engine-condition ranges. These ranges may have been previously obtained by dividing the entire internal-combustion-engine conditions into a plurality of engine-condition ranges, the dividing depending on intake-air pipe pressures occurring during the engine combustion operation. The above-mentioned learning values may be corrected by varying (updating) them according to the variations of the characteristics in the engine due to the prolonged passage of time. This correction of the learning values may be achieved using the variation ranges of the above-mentioned FAF.
The Japanese Utility-Model Laid-Open Application No. 61-206262 discloses an air-fuel ratio control system such as mentioned above. The disclosed system has a solenoid valve located in a passage connecting the canister and the intake-air passage provided downstream of the throttle valve. This solenoid valve is used for cutting off the connection passage by closing the solenoid valve and used for connecting the canister and the intake-air passage by opening the valve. The solenoid valve is in its open condition while the engine is in a particular operation condition. This opening of the solenoid valve causes the evaporated fuel stored in the canister to be purged to the intake-air passage.
This purging causes the above-mentioned FAF to vary accordingly because the air to fuel ratio sensor detects the increase in the amount of fuel. However this variation of FAF does not result from the above-mentioned variation in the engine characteristics due to the prolonged passage of time. Thus, it is needed to eliminate the variation of FAF affecting the learning values. To eliminate these effects, the updating of the learning values is to be executed using the variation of FAF occurring while the solenoid valve is closed so as to stop the purging of the evaporated fuel (This action will be referred to as "purge cut" hereinafter).
Drawbacks in the above-mentioned system will now be described. As mentioned above, the purge cut needs to be executed so as to update the learning values. The more times the learning values are updated in response to possible engine characteristics variation due to the prolonged passage of time, the more appropriately and thus the more efficiently the air-fuel ratio control operation may be executed. However, updating the learning-value may result accordingly in the purge cut needing execution many times. This repeated purge-cut reduces the evaporated fuel quantity being purged from the canister and thus causes problems such as excess storing of the evaporated fuel in the canister, which excess storing may cause leaking of the evaporated fuel to the atmosphere. Such problems may become serious problems when an automobile has been stopped for a long time under an excessively hot atmospheric condition.